When a beam of laser propagates in a crystal with non-zero components of the second order polarizability tensor, the crystal will produce NLO effects such as second harmonic generation (SHG), sum-frequency generation (SFG), difference-frequency generation (DFG) and parametric amplification (OPA). NLO devices such as second harmonic generators, up and down frequency converter and parametric oscillator can be prepared using crystals having NLO properties (Please refer to U.S. Pat. Nos. 3,262,058, 3,328,723, 3,679,907, 3,747,022, 3,949,323 and 4,826,283).
Second harmonic generation (SHG) is the most important NLO effect. An electromagnetic wave with frequency of w propagating in a NLO crystal will induce a polarization wave of a frequency of 2 w. That is the so-called "SECOND HARMONIC GENERATION". The conversion efficiency of a SHG crystal is proportional to the effective SHG coefficient (d.sub.eff) square and the input laser power, and is also relative with the phase-matching condition. When other conditions are selected, if phase matching is achieved, the conversion efficiency will reach the maximum. Generally, there are two types of phase-matching: Type I wherein the two incident waves have the same polarization; and Type II wherein the two waves have orthogonal polarization. The most-commonly used method for achieving phase-matching is to find a suitable orientation of the crystal as the propagating orientation of incident waves, and along this orientation, the refractive indices are the same for both the fundamental and the second harmonic waves. When the orientation of the crystal drifts from this special orientation, phase-mismatching will generally occur. The value of acceptance angle of a crystal reflects the affecting extent on the conversion efficiency when the acceptance angle drifts from the phase-matching condition. In addition, due to the influence of the double refraction of crystal which results in the difference between the energy propagation direction and the phase direction, the fundamental wave and second harmonic wave will separate each other after they propagate in the crystal for a certain distance. That is so-call WALK-OFF effect. The walk-off angle restricts the length of crystal having effective function.
Desirable NLO crystals should have the following requisites: great nonlinear polarization coefficient; wide transparency range; good phase-matching condition and high damage threshold; easy to grow; and easy to obtain a single crystal with large dimension and high quality.
BBO (barium betaborate, low temperature modification: B-BaB.sub.2 O.sub.4) and LBO (lithium triborate: LiB.sub.3 O.sub.5) are excellent NLO crystals of borates developed in recent years, and have been used widely in NLO devices, especially in NLO devices which can stand up to lasers with high power density (See Scientia, Sinica, B28, 235, 1985; U.S. Pat. No. 4,826,283 and Chinese Patent No. 88102084). It has been found that BBO has good UV transparency ability (UV absorbing edge is 190 nm); high damage threshold (15 GW/cm.sup.2, 0.1 ns, 1064 nm); and great effective SHG coefficient (about 6 times of that of KDP). The main disadvantage of BBO is that the z component of its SHG coefficients is too small (d.sub.31 &lt;0.07 d.sub.11) to restrict its use in deep UV range and in laser systems of larger divergence. In addition, because of acceptance angle of BBO (&lt;1 mrad-cm) is too small, high working accuracy is needed for it.
The UV transparent ability of LBO (UV absorption edge is 160 nm) is the best and the damage threshold of LBO SINGLE CRYSTAL (25 GW/cm.sup.2, 0.1 ns, 1064 nm) is the highest among the NLO crystals. However, LBO is an incongruent compound which must be prepared by flux method, the period of its production is much longer (over 1 month), the yield is low, and the cost for production is high.
J. Krogh-Moe first reported the crystal structure of cesium triborate, CsB.sub.3 O.sub.5 (Acta Crystallography, Vol. 13, 889-892, 1960; and Acta Crystallography, Vol. B30, 1178-1180, 1974). It crystallizes in the space group P2.sub.1 2.sub.1 2.sub.1, and is a biaxial crystal. The largest crystalline size reported was only 0.10.times.0.17.times.0.046 mm.sup.3. A. J. Marlor et. al. studied the crystallization kinetics of CsB.sub.3 O.sub.5 from its undercooled melt using microscope (Physics and Chemistry of Glasses, Vol. 16, 108-14 111, 1975). However, neither a single crystal of CsB.sub.3 O.sub.5 with a size large enough for examining its physical properties, nor devices made of CsB.sub.3 O.sub.5 have ever been reported until now.